There is used a semiconductor power amplifier using a compound semiconductor for a microwave or millimeter-wave power amplifier instead of a magnetron oscillator. However, the semiconductor power amplifier is not necessarily excellent in power gain of an individual semiconductor amplifier such as a transistor, and thus a plurality of semiconductor amplifiers are cascaded to obtain a desired power gain.
When a plurality of amplifiers are cascaded, however, stability in the power amplifier is deteriorated, for example, an oscillation easily occurs due to a state change in the amplifier caused by a reflective wave due to mismatch with a load to be connected.
Such a problem is conspicuous when a load impedance changes over time such as when an object is heated or processed by use of a microwave amplified by a semiconductor power amplifier, for example.
Thus, conventionally, a circulator or attenuator was provided between cascaded semiconductor amplifiers, thereby converting a reflective wave into thermal energy for absorption.
However, when an attenuator is inserted, power gain or power efficiency is deteriorated. A circulator has a small impact on power gain or power efficiency, but has a problem that it does not function under a ferromagnetic field.